Process for continuous casting and vacuum degasification



Aug. 28, 1962 F. w. CHAMBERS 3,050,798

PROCESS FOR CONTINUOUS CASTING AND VACUUM DEGASIFICATION Filed June 15, 1960 INVENTOR.

Frank W. Chambers ATTORNEY United States Patent 3,050,798 PROCESS FOR CGNTINUOUS CASTING AND VACUUM DEGASEFICATION Frank W. Chambers, Darien, (301111., assiguor, by mesne assignments, to Strategic Materials Corporation, New

York, N.Y., a corporation of New York Filed June 13, 1960, Ser. No. 35,486 7 Claims. (Cl. 22-2091) This invention relates generally to metallurgy and more particularly to a process for the continuous casting of various types of metals and alloys. In particular, the invention involves the provision of a process in which vacuum conditions are utilized both to control the casting process and to remove harmful impurities from the molten metal as it is cast.

In the gradual development of continuous casting in this country; starting with US. Patent No. 1,908, issued in 1840 and continuing to the present date, every conceivable direction or angle of casting has been explored, and, to this end, the types of molds utilized have included sliding, oscillating, vibrating, wheel, chain, liquid, and centrifugal, formed of such diverse materials as copper, graphite, refractories, mercury and salt, among others. Even a cursory review of the literature in this field reveals that While a great deal of attention has been directed to what happens to the metal once it is in the mold (or tundis as it is commonly called), very little work has gone into getting the metal there, many processes relying on the simple expedient of a man with a lever attached to a conventional launder, periodically filling the mold as metal is withdrawn.

in any continuous process, it is axiomatic that the rate of input of materials is proportional to the production rate, the proportionality factor taking into account the reactions and changes or state that the materials undergo in the process. In the continuous casting of metals, however, the most important factors controlling production rate are the size of the cross-section being cast, heat transfer characteristics of the material, rate of heat withdrawal from the material, and mechanical facilities for mold movement and product removal. While a great deal of effort has gone into establishing production rates based on these factors,l-ittle or no effort has been made to correlate the production rate so computed with the rate of input of molten metal, leaving this factor to the arbitrary discretion of the aforementioned man with a lever.

it is one object 'of my invention to provide means for eifecting automatic metering of mol en metal into a continuous casting tundish that is directly and automatically correlated with the rate of removal of the solid, cast metal.

The attitude that continuous casting commenced after the metal was delivered to the tundish, also presupposes that the metal is of a proper composition and temperature for casting. During the past decade, however, research has established that while normal smelting and refining processes produce metal of proper specification with respect to major constituents and impurities, they accomplish little towards removing interstitial impurity gases such as hydrogen, oxygen, and nitrogen. Since it is now well establishedthat these interstitial impurities can have such deleterious efiects as embrittlement, crack initiation, and delayed failure, a great number of processes have been developed to effect the removal of these elements. Most of the processes so developed to date involve some sort of vacuum treatment, either pouring from one ladle to another under vacuum conditions, actual casting of ingots under vacuum, or in the case of highly reactive spaceage metals such as titanium, zirconium, and hafnium, carrying out the entire refining process under a high vacuum.

3,050,798 Patented Aug. 28, 1962 "ice Since all of these vacuum treatments are inherently batch-type discontinuous operations, and since they also involve a temperature drop in the molten metal, there has been little eifort made to adapt vacuum degassing operations to continuous casting.

It is a further object of my invention to provide. for continuous degassing of the metal stream as it flows into the tundish of a continuous casting unit. It is a still further object to provide a process for making continuous cast metal products substantially free of interstitial gaseous impurities such as hydrogen, oxygen and nitrogen.

In essence, my invention consists of a continuous castting unit equipped with a vacuum bell, thetundish being fed by an open trough leading from a siphon that extends through the side of the bell into a feed-control ladle. In operation, metal from the ladle is drawn up through the siphon by means of the vacuum inside the bell and delivered to the tundish through the open trough. The rate of delivery of metal into the tundish is directly proportional to the degree of vacuum maintained in the bell, and is set to correspond to the rate of removal of the cast metal from the bottom of the tundish. A Teflon (trademark) collar connects the tundish support with the vacuum-bell support, allowing the necessary oscillation of the tundish While still maintaining the desired degree of vacuum. While the metal is traversing the open trough and falling into the tundish it is in direct contact with the reduced pressure maintained inside the bell, and gases such as hydrogen, oxygen, and nitrogen contained in the metal are immediately drawn into the atmosphere and thence into the vacuum manifold system.

It is believed that the process and apparatus of my invention may be best understood by referring to the figure which shows a side view, partly in section, of one embodiment of my invention.

With reference to the figure, the vacuum bell 1, which can be made of Pyrex (trademark) for small installations but will of necessity have to be of welded steel construetion or other material with suitable viewing ports on larger installations,isconnected to a vacuum exhaust manifold 2 which is in turn connected to vacuum pumps (not shown) of suitable capacity for maintaining a pressure in the range of 1 mm. Hg. Thevacuum-bell support 3 is independent of the tundish support 4, as, in most continuous casting operations, it is necessary for the tundish to oscillate vertically. Vacuum is maintained between the two while still allowing the necessary oscillation by means of the Teflon (trademark) collar 5. The tundish 6 is of any suitable design and material for casting the desired metal. Molten metal 7 flows into the tundish from an open trough 8 which is merely an extention of the tubular siphon 10. The siphon enters the vacuum bell through a hole provided with a vacuum-tight seal 9, and outside of the bell extends down well below the level of molten metal in the feed-control ladle 11. On relatively small installations, the feed-control ladle is of moderate size and is periodically filled with hot metal from a furnace by means of a launder 13, it being necessary only to keep the level of metal in the ladle above the bottom of the siphon at all times, lest the siphon suck in air and destroy the vacuum. In larger installations, where it would be inconvenient to have the unit adjacent to a furnace, the feed control ladle would be as large as necessary, and provided with means for maintaining the molten metal at a proper pouring temperature. In this case the siphon would either have to extend nearly to the bottom of the ladle, or the ladle would have to be raised as the metal was drawn out.

Since continuous casting is not an inherently fast operation, it is desirable that the metal in the feed control ladle be held under an atmosphere of a reducing gas so that a heavy oxide layer will not be formed on the surface A simple hood arrangement 12 is provided for this ,purpose; however the reducing gas need not be at more than atmospheric pressure, as its main function is to purge the air above the molten metal. I

In operation, it is generally advisable to purge the bell chamber with an inert gas such as. argon before casting is commenced. Once the proper vacuum has been established, casting beginsQthe difference in pressure over the metal in the ladle and inside the bell drawing metal up through the siphon and into the tundish. The rate at which metal flows through the siphon is'dependent on the degree of vacuum maintained inside the bell chamber, and it is an easy matter to set the vacuum equipment so that a coinstant rate of delivery of metal into the tundish is maintained. This, of course, is equal to the rate of 'metal withdrawal from the bottom of the tundish, so that the level of metal in the tundish is maintained substantially constant. Degassification of the liquid metal occurs as it emergesfrom the tubular portion of the siphon into the trough-shaped portion 8, and continues until the metal is submerged in the tundish.

Having thus described the subject matter of my invention, What it is desired to secure by Letters Patent is:

1. Process for the continuous casting and vacuum degassing of metals and alloys that comprises maintaining a vacuum chamberat a substantially subatmospheric pressure while maintaining an exteriorly located molten metal bath at substantially'atmospheric pressure, drawing said molten metal into said vacuum chamber through a siphon terminating in said vacuum chamber and communicating with the interior of said molten metal bath, said siphon being adapted to deliver said metal to an oscillating continuous-casting mold located within said vacuum chamber, allowing said molten metal to solidify in said mold and said subatmospheric pressure controlling the rate of flow of said metal from said bath through said siphon and into said mold, and continuously recovering said solidified metal therefrom. e

2. The process as claimed in claim 1, wherein said sub-- atmospheric pressure acts to remove entrained gaseous impurity elements from said molten metal as it passes from said siphon into said mold.

3. The process as claimed in claim 2, wherein said subatmospheric pressure is of the order of one millimeter (1 mm.) of mercury.

4. Process for the production of continuously-cast while the metal is being drawn out through the siphon.

metal and alloy products substantially free of entrained gaseous impurity elements that comprises maintaininga vacuum chamber at a substantially constant subatmospheric pressurewhile maintaining an exteriorly located molten metal bath at substantially atmospheric pressure, drawing said molten metal into said vacuum chamber through a siphon terminating in said vacuum chamber and communicating with the interior of said molten metal 7 bath, said siphon being adapted to deliver said metal to an oscillating continuous-casting mold located Within said vacuum chamber, said subatmospheric pressure acting to removed entrained gaseous impurity elements from said molten metal as-it is delivered to said mold and also acting to control the rate of flow of said metal from said bath through said siphon and into said mold, f-allowing the degassified molten metal to solidify in said mold, and continuously withdrawing a degassified metal product therefrom.

5. The process as claimed in claim 4, wherein said subatmospheric pressure is of the order of one millimeter (1 mm.) of mercury.

6. Process for the production of continuously-cast metal and alloy products substantially free of entrained hydrogen, oxygen and nitrogen'that comprises maintaining a vacuum chamber at a substantially constant subatmospheric pressure while maintaining an'exteriorly located molten metal bath at substantially atmospheric:

pressure, drawing said molten metal into said vacuum chamber through a siphon terminating in said 'vacuum chamber and communicating with the interior of said molten metal bath, said siphon being adapted to deliver said metal to an oscillating continuous-casting mold located within said vacuum chamber, said subatmospheric pressure acting to remove entrained hydrogen, oxygen, and nitrogen from said molten metal as it is delivered to said mold and also acting to control the rate of flow of said metal from said bath through said siphon and into said mold, allowing the degassified molten metal to solidify in said mold, and'continuously withdrawing a degassified solidified metal product therefrom.

7. The process as claimed in claim 6, wherein said subatmospheric pressure is of the order of one millimeter (1 mm.) of mercury.

References Cited in the file of this patent UNITED STATES PATENTS 2,548,696 Barstow et a1. Apr. 10, 1951 2,825,104 Jones Mar. 4, 1958 2,837,790 Rozian June10, 1958 2,873,491 Brennan Feb. 17, 1959 

